The present invention relates to a graded-refractive-index optical plastic material (hereinafter sometimes referred to as optical plastic material for short) having high transparency and high heat resistance simultaneously, which conventional optical plastics hardly ever attained, and a method for its production.
The optical plastic material of the present invention may be a light transmission medium such as an optical fiber, or a body material of a light transmission medium such as a preform of an optical fiber.
A light transmission medium which is the optical plastic material of the present invention is free from light scattering and very transparent to light at wavelengths within a wide range from ultraviolet light to near infrared light, since it is made of an amorphous resin, therefore it is useful for optical systems for light of various wavelengths. In particular, the optical plastic material of the present invention provides a light transmission medium with small losses at wavelength of 1300 nm and 1550 nm, at which a trunk vitreous silica fiber is used in the field of optical communication.
A light transmission medium which is the optical plastic material of the present invention has heat resistance, chemical resistance, humidity resistance and nonflammability enough to withstand severe use conditions, for example, in an engine room of an automobile.
A light transmission medium which is the optical plastic material of the present invention is useful as various graded-refractive-index light transmission medium such as a graded-refractive-index optical fiber, a rod lens, an optical waveguide, an optical decoupler, a wavelength multiplexer, a wavelength demultiplexer, an optical attenuator, a light switch, an optical isolator, a light transmitting module, an light receiving module, a coupler, an optical deflector and an optical integrated circuit. Graded-refractive-index means a region wherein the refractive index of a light transmission medium varies continuously in a specific direction. For example, a graded-refractive-index optical fiber shows a refractive index profile that the refractive index parabolically decreases from the center of the fiber along the radii.
When the optical plastic material of the present invention is a body material of a light transmission medium, it is spun, for example, by hot drawing to prepare a light transmission medium such as a graded-refractive-index optical fiber.
Heretofore, as resins for graded-refractive-index plastic light transmission medium, optical plastics represented by methyl methacrylate resins, tetrafluoroethylene resins disclosed in WO94/04949 and vinyl fluoride resins have been proposed.
With respect to stepped-refractive-index plastic optical fibers, many proposals have been made to use optical plastics such as a methyl methacrylate resin, a styrene resin, a carbonate resin and a norbornene resin for a core and a fluoropolymer cladding. Japanese Unexamined Patent Publication No. 244007/1990 proposes use of a fluoropolymer core and a fluoropolymer cladding.
The present invention provides an optical plastic material having heat resistance, humidity resistance, chemical resistance and nonflammability required for applications to an automobile, an office automation (OA) equipment, an electrical appliance and the like, which light transmission medium made of a methyl methacrylate resin or a norbornene resin have never attained.
Further, the object of the present invention is to provide a novel optical plastic material which is useful for ultraviolet light (wavelength from 200 nm to 400 nm) and near infrared light (wavelength from 700 nm to 2500 nm), which are unavailable to light transmission medium made of a methacrylate resin, a carbonate resin and a norbornene resin, and has low light transmission losses in a wide transmission zone and a method of its production.
The present inventors have conducted extensive researches taking the above-mentioned problems into consideration, and consequently found that a fluoropolymer which substantially has no Cxe2x80x94H bond is the most suitable to provide heat resistance, humidity resistance, chemical resistance and nonflammability and eliminate Cxe2x80x94H bonds (namely carbon-hydrogen bonds) which absorb near infrared light. The fluoropolymer has Cxe2x80x94F bonds (namely carbon-fluorine bonds) instead of Cxe2x80x94H bonds.
When a substance is exposed to light, a certain interatomic bond absorbs preferentially light of wavelength resonant with its stretching vibration and deformation vibration. Conventional polymeric materials used for plastic optical fibers are mostly compounds having Cxe2x80x94H bonds. Such polymeric materials which basically have Cxe2x80x94H bonds show the main absorption bands at a shorter wavelength (3400 nm) in the infrared region, since a hydrogen atom is so light as to easily vibrate. Accordingly, in the near infrared to infrared region (from 600 to 1550 nm), which is the wavelength region of a light source, relatively lower harmonic absorption peaks due to the stretching vibration of Cxe2x80x94H bonds appears at intervals and they are greatly responsible for absorption loss.
If hydrogen atoms are substituted by fluorine atoms, these harmonic absorption peaks shift to longer wavelengths, and the amount of absorption in the near infrared region decreases. In the case of a PMMA (polymethyl methacrylate) having Cxe2x80x94H bonds, the absorption loss attributable to the Cxe2x80x94H bonds is estimated theoretically to be 105 dB/km at a wavelength of 650 nm and at least 10000 dB/km at a wavelength of 1300 nm.
On the contrary, in the case of a material in which hydrogen atoms are substituted with fluorine atoms, there is substantially no absorption loss at a wavelength of 650 nm, and the absorption loss at a wavelength of 1300 nm, which is between the sixth and the seventh overtones, is in the order of 1 dB/km and therefore negligible. For this reason, we propose to use a compound having Cxe2x80x94F bonds.
It is also preferred to eliminate functional groups such as a carboxyl group and a carbonyl group which inhibit heat resistance, humidity resistance, chemical resistance and nonflammability. Further, since the presence of a carboxyl group results in absorption of near infrared light, and the presence of a carbonyl group results in absorption of ultraviolet light, it is preferred to eliminate these groups. In addition, in order to reduce a transmission loss due to light scattering, it is important to use an amorphous polymer.
In the case of a stepped-refractive-index optical fiber, multimodal light is propagated in it, by being reflected on the interface between the core and the cladding. Therefore, mode dispersion occurs, and as a result, the transmission zone decreases. However, a graded-refractive-index optical fiber hardly causes mode dispersion, and therefore, the transmission zone increases.
The present inventors found out an optical plastic material composed of an amorphous fluoropolymer which substantially has no Cxe2x80x94H bond, especially a fluoropolymer having a cyclic structure on its main chain, and a material which differs from the polymer in refractive index, wherein the concentration of the material shows a gradient in a specific direction and a method of its production for the first time, and achieved the following present inventions (1) to (2).
(1) A graded-refractive-index optical plastic material composed of an amorphous fluoropolymer (a) which substantially has no Cxe2x80x94H bond and at least one material (b) which differs from the fluoropolymer (a) in refractive index by at least 0.001, wherein the material (b) is so distributed in the fluoropolymer (a) as to have a concentration gradient in a specific direction.
(2) A method of producing a graded-refractive-index optical plastic material, which comprises melting an amorphous fluoropolymer (a) which substantially has no Cxe2x80x94H bond, injecting at least one material (b) which differs from the fluoropolymer (a) in refractive index by at least 0.001, or the fluoropolymer (a) containing the material (b) at the center of the melt of the fluoropolymer (a), and molding the melt while or after diffusing the material (b) to form a region wherein the refractive index varies continuously. Heretofore, a tetrafluoroethylene resin, a perfluoro(ethylene-propylene)resin, a perfluoroalkoxy resin, a vinylidene fluoride resin, an ethylene-tetrafluoroethylene resin, a chlorotrifluoroethylene resin and the like have been widely known as fluoropolymers. However, since these fluoro resins are crystalline and causes light scattering, they are poor in transparency and unfavorable as materials for a plastic light transmission medium.
In contrast to these fluoro resins, amorphous fluoropolymers are free from light scattering due to crystals and therefore, are excellent in transparency. The fluoropolymer (a) of the present invention is by no means limited so long as it is an amorphous fluoropolymer having no Cxe2x80x94H bond, however, a fluoropolymer having a cyclic structure on its main chain is preferred. As a fluoropolymer having a cyclic structure on its main chain, fluoropolymers having a fluorine-containing alicyclic structure, a fluorine-containing cyclic imide structure, a fluorine-containing triazine ring structure or a fluorine-containing aromatic ring structure are preferred. Among fluoropolymers having a fluorine-containing alicyclic structure, those having a fluorine-containing alicyclic ether structure are more preferred.
A fluoropolymer having a fluorine-containing alicyclic structure is less likely to undergo orientation of polymer molecules, when hot drawn or melt spun into fibers, as compared with fluoropolymers having a fluorine-containing cyclic imide structure, a fluorine-containing triazine ring structure or a fluorine-containing aromatic ring structure. Consequently, it does not cause light scattering, therefore is a more preferred polymer.
The viscosity of the fluoropolymer (a) in a molten state is preferred to be from 103 to 105 poise at a melt temperature of from 200xc2x0 C. to 300xc2x0 C. If the melt viscosity is too high, not only melt spinning is difficult, but also diffusion of the material (b) required for a graded refractive index, hardly takes place, and formation of a graded refractive index is difficult. On the other hand, if the melt-viscosity is too low, there are practical problems. Namely, in the case of use as a light transmission medium in an electronic equipment or an automobile, it becomes soft upon exposure to a high temperature, and the light transmission performance becomes poor.
The number-average molecular weight of the fluoropolymer (a) is preferably from 10,000 to 5000,000, more preferably from 50,000 to 1000,000. A too small molecular weight can interfere with heat resistance, and too a large molecular weight makes it difficult to form a graded-refractive-index light transmission medium, such being unfavorable.
As a polymer having a fluorine-containing alicyclic structure, those obtainable by polymerization of a monomer having a fluorine-containing cyclic structure, and a polymer having a fluorine-containing alicyclic structure on its main chain which is obtainable by cyclic polymerization of a fluorine-containing monomer having at least two polymerizable double bonds are preferred.
Polymers having a fluorine-containing alicyclic structure on their main chains which are obtainable by polymerization of monomers having a fluorine-containing alicyclic structure are reported by Japanese Examined Patent Publication No. 18964/1988 and the like. Namely, polymers having a fluorine-containing alicyclic structure or their main chains are obtained by homopolymerization of a monomer having a fluorine-containing alicyclic structure such as perfluoro(2,2-dimethyl-1,3-dioxole), or by polymerization of such a monomer with a radical polymerizable monomer such as tetrafluoroethylene, chlorotrifluoroethylene and perfluoro(methyl vinyl ether).
Further, polymers having a fluorine-containing alicyclic structure on their main chains which are obtainable by cyclic polymerization of a fluorine-containing monomer having at least two polymerizable double bonds are reported by Japanese Unexamined Patent Publication No. 238111/1988, Japanese Unexamined Patent Publication No. 238115/1988 and the like. Namely, polymers having a fluorine-containing alicyclic structure on their main chains are obtained by cyclic polymerization of a monomer such as perfluoro(allyl vinyl ether) and perfluoro(butenyl vinyl ether), or copolymerization of such a monomer with a radical polymerizable monomer such as tetrafluoroethylene, chlorotrifluoroethylene and perfluoro(methyl vinyl ether).
Polymers having a fluorine-containing alicyclic structure on their main chains are also obtained by copolymerization of a monomer having a fluorine-containing alicyclic structure such as perfluoro(2,2-dimethyl-1,3-dioxole) with a fluorine-containing monomer having at least two polymerizable double bonds such as perfluoro(allyl vinyl ether) and perfluoro(butenyl vinyl ether).
As examples of the above-mentioned polymer having a fluorine-containing alicyclic structure, those having a repeating unit selected from the following formulae (I) to (IV) are mentioned. Part of the fluorine atoms in such polymers having fluorine-containing alicyclic structure may be substituted with chlorine atoms for the purpose of increase in refractive index. 
[in the above formulae (I) to (IV), l is from 0 to 5, m is from 0 to 4, n is from 0 to 1, l +m+n is from 1 to 6, each of o, p and q is from 0 to 5, o+p+q is from 1 to 6, R is F or CF3, R1 is F or CF3, R2 is F or CF3, X1 is F or Cl, and X2 is F or Cl].
As the polymer having a fluorine-containing alicyclic structure, polymers having a cyclic structure on their main chains are preferred. Those containing a polymeric unit having a cyclic structure in an amount of at least 20 mol %, preferably at least 40 mol % are preferred in view of transparency and mechanical properties.
The material (b) is at least one material which differs from the fluoropolymer (a) in refractive index by at least 0.001. It may have a higher refractive index or a lower refractive index than the fluoropolymer (a). In optical fibers, it is usual to use a material having a higher refractive index than the fluoropolymer (a).
As the material (b), low-molecular weight compounds, oligomers and polymers containing an aromatic ring such as a benzene ring, a halogen atom such as chlorine, bromine or iodine, or a bonding group such as an ether bond are preferred. Further, the material (b) is a material which substantially has no Cxe2x80x94H bond for the same reason as the fluoropolymer (a). The difference in refractive index between the fluoropolymer (a) and the material (b) is preferably at least 0.005.
The oligomeric or polymeric material (b) may be an oligomer or a polymer of such a monomer constituting the fluoropolymer (a) as described above, which differs from the fluoropolymer (a) in refractive index by at least 0.001. Such a monomer is selected from those which form a polymer which differs from the fluoropolymer (a) in refractive index by at least 0.001. For example, it is possible to use two kinds of fluoropolymers (a) having different refractive indices and distribute one polymer (a) in the other polymer (a), as the material (b).
The material (b) preferably has a solubility parameter within 7(cal/cm3)xc2xd of that of the matrix. A solubility parameter is a property value which is a measure of the miscibility between materials. The solubility parameter xcex4 is represented by the formula xcex4=(E/V)xc2xd, wherein E is the cohesive energy of a molecule of material, and V is the molar volume.
As a low-molecular weight compound, halogenated aromatic hydrocarbons having no hydrogen atom bonded to a carbon atom may be mentioned. Halogenated aromatic hydrocarbons containing only fluorine atoms as halogen atoms, and halogenated aromatic hydrocarbons containing a fluorine atom and another halogen atom are preferred in view of the miscibility with the fluoropolymer (a). Among such halogenated aromatic hydrocarbons, those having no functional group such as a carbonyl group or a cyano group are more preferred.
As such a halogenated aromatic hydrocarbon, a compound represented by the formula "PHgr"r-Zb [wherein "PHgr"r is a b valent fluorinated aromatic ring residue having fluorine atoms substituted for all the hydrogen atoms, and Z is a halogen atom other than fluorine, xe2x80x94Rf, xe2x80x94COxe2x80x94Rf, xe2x80x94Oxe2x80x94Rf or xe2x80x94CN, wherein Rf is a perfluoroalkyl group, a polyfluoroperhaloalkyl group or a monovalent "PHgr"r, and b is 0 or an integer of at least 1] may, for example, be mentioned. As the aromatic ring, a benzene ring or a naphthalene ring may be mentioned. The carbon number of a perfluoroalkyl group or a polyfluoroperhaloalkyl group as Rf is preferably at most 5. As a halogen atom other than fluorine, a chlorine atom and a bromine atom are preferred.
As specific compounds, for example, 1,3-dibromotetrafluorobenzene, 1,4-dibromotetrafluorobenzene, 2-bromotetrafluorobenzotrifluoride, chloropentafluorobenzene, bromopentafluorobenzene, iodopentafluorobenzene, decafluorobenzophenone, perfluoroacetophenone, perfluorobiphenyl, chloroheptafluoronaphthalene and bromoheptafluoronaphthalene may be mentioned.
As the polymeric or oligomeric material (b) among those having the above-mentioned repeating units (I) to (IV), fluoropolymers having a different refractive index from the fluoropolymer (a) to be used in combination (for example, a combination of a fluoropolymer containing fluorine atoms only as halogen atoms with a fluoropolymer containing fluorine atoms and chlorine atoms, and a combination of two kinds of fluoropolymers obtained by polymerizing at least two monomers of different kinds in different proportions) are preferred.
Further, in addition to the above-mentioned fluoropolymers having a cyclic structure on their main chains, oligomers of monomers containing no hydrogen atom such as tetrafluoroethylene, chlorotrifluoroethylene, dichlorodifluoroethylene, hexafluoropropylene and perfluoroalkyl vinyl ether, and co-oligomers of at least two of these monomers may be used as the material (b). Further, perfluoropolyethers having a structural unit xe2x80x94CF2CF(CF3)Oxe2x80x94 or xe2x80x94(CF2)nOxe2x80x94 (wherein n is an integer of from 1 to 3) may be used. The molecular weights of the oligomers are selected within such a range of molecular weight that they are amorphous, and are preferably from 300 to 10,000 in terms of number-average molecular weight. In view of easy diffusion, it is more preferred that the number-average molecular weights are from 300 to 5000.
The particularly preferable material (b) is a chlorotrifluoroethylene oligomer since it is excellently compatible with the fluoropolymer (a), particularly with a fluoropolymer having a cyclic structure on its main chain. By virtue of its good compatibility, it is possible to easily mix the fluoropolymer (a), particularly the fluoropolymer having a cyclic structure on its main chain with a chlorotrifluoroethylene oligomer by hot-melting them at 200 to 300xc2x0 C. It is also possible to homogeneously mix them by dissolving them in a fluorine-containing solvent and then removing the solvent. The preferred molecular weight of a chlorotrifluoroethylene oligomer is from 500 to 1500 in terms of the number-average molecular weight.
The optical plastic material of the present invention is most preferably a graded-refractive-index optical fiber. In the optical fiber, the material (b) is so distributed in the fluoropolymer (a) as to have a concentration gradient in the direction of from the center to the periphery. Preferably, it is an optical fiber wherein the material (b) is a material having a higher refractive index than the fluoropolymer (a), and the material (b) is so distributed as to have such a concentration gradient that the concentration of the material (b) decreases in the direction of from the center of the optical fiber to the periphery. In some cases, an optical fiber wherein the material (b) is a material having a lower refractive index than the fluoropolymer (a), and the material (b) is so distributed as to have a concentration gradient that the concentration of the material (b) decreases in the direction of from the periphery of the optical fibers the center, is also useful. A light transmission medium such as the former optical fiber is usually produced by arranging the material (b) at the center and diffusing the material (b) toward the periphery. A light transmission medium such as the latter optical fiber is produced by diffusing the material (b) from the periphery toward the center.
A light transmission medium which is the optical plastic material of the present invention has a transmission loss per 100 m of at most 1000 dB at wavelengths of from 700 to 1,600 nm. Particularly, when a fluoropolymer having an alicyclic structure on its main chain is used, it has a transmission loss per 100 m of at most 50 dB. It is quite advantageous that the transmission loss is at such a low level at relatively long wavelengths of from 700 to 1,600 nm. Namely, it has advantages that since it is available to the same wavelength as vitreous silica optical fiber, it can be connected to a vitreous silica optical fiber without any difficulties, and that a cheaper light source can be used as compared with the case of conventional plastic optical fibers which are available only to light at wavelengths shorter than from 700 to 1,600 nm.
In production of the optical plastic material of the present invention, the molding of the resins and the formation of the graded refractive index may be carried out simultaneously or separately. For example, the optical plastic material of the present invention may be so produced by spinning or extrusion molding that a graded refractive index is formed at the same time as formation of a graded refractive index. It is also possible to form a graded refractive index after molding the resins by spinning or extrusion molding. Further, it is possible to produce a preform (body material) having a graded refractive index and then form (for example spin) the preform into an optical plastic material such as an optical fiber. As described above, the optical plastic material of the present invention also means such a preform having a graded refractive index.
As a method of producing the optical plastic material of the present invention, for example, the following methods (1) to (7) may be mentioned. However, the present invention is not limited to these methods. The method (1) is particularly preferred.
(1) A method which comprises melting the fluoropolymer (a), injecting the material (b) or a fluoropolymer (a) containing the material (b) at the center of the melt of the fluoropolymer (a), and then molding the melt while or after diffusing the material (b).
In this case, the material (b) may be injected at the center not only so as to form only one layer but also so as to form multiple layers. The molding is carried out by melt-extrusion, which is suitable for forming a rod-like body material such as a preform of an optical fiber, or by melt-spinnig, which is suitable for forming an optical fiber.
(2) A method which comprises dip-coating the material (b) or the fluoropolymer (a) containing the material (b) on a core formed from the fluoropolymer (a) by melt spinning or drawing.
(3) A method which comprises forming a hollow tube of the fluoropolymer (a) by using a rotating glass tube or the like, filling in the polymer tube with a monomer phase which gives the material (b) or the fluoropolymer (a) which contains the material (b), and then polymerizing the monomer phase while rotating the polymer tube at a low speed.
In the case of interfacial gel polymerization, at the polymerization step, the tube of the fluoropolymer (a) swells up in the monomer phase and forms a gel phase, and the monomer molecules are polymerized while preferentially diffusing in the gel phase.
(4) A method wherein two kinds of monomers with different reactivities, one of which is a monomer which forms the fluoropolymer (a), and the other is a monomer which forms the material (b), are used, and the polymerization reaction is carried out so that the compositional proportion of the resulting fluoropolymer (a) to the resulting material (b) varies continuously in the direction from the periphery to the center.
(5) A method which comprises hot-drawing or melt-extruding a mixture of the fluoropolymer (a) and the material (b) obtained by homogeneously mixing them or by homogeneously mixing them in a solvent and then removing the solvent upon evaporation, into fibers, and then (or immediately after the formation of the fibers) bringing the fibers into contact with an inert gas under heating to evaporate the material (b) from the surface and thereby forming a graded refractive index. Or, a method wherein after the formation of the fibers, the fibers are immersed in a solvent which does not dissolve the fluoropolymer (a) but dissolves the material (b) so as to dissolve out the material (b) from the surface of the fibers so that a graded refractive index is formed.
(6) A method which comprises coating a rod or a fiber of the fluoropolymer (a) with only the material (b) which has a smaller refractive index than the fluoropolymer (a) or with a mixture of the fluoropolymer (a) and the material (b), and then diffusing the material (b) by heating to form a graded refractive index.
(7) A method which comprises mixing a high-refractive-index polymer and a low-refractive-index polymer by hot-melting or in a state of a solution containing a solvent, and diffusing them in each other while (or after) multilayer-excluding in a state that each has a different mixing ratio, to eventually obtain a fiber having a graded refractive index. In this case, the high-refractive-index polymer may be the fluoropolymer (a), and the low-refractive-index polymer may be the material (b). The high-refractive-index polymer is the material (b), and the low-refractive-index polymer is the material (b).
In the present invention, by virtue of the application of an amorphous fluoro resin to various plastic light transmission medium such as a graded-refractive-index optical fiber, a graded-refractive-index optical waveguide and a graded-refractive-index rod lens, it is possible to transmit light ranging from ultraviolet light to near infrared light with a quite low loss.
A graded-refractive-index optical fiber is particularly suitable for optical communication over short distances in spite of its large diameter since it is flexible and it is easy to form branches and junctions or it. However, no practical optical fiber with a low loss has been proposed so far. The present invention provides a practical optical fiber with a low loss for optical communication over short distances.
The light transmission medium of the present invention provides a plastic light transmission medium having heat resistance, chemical resistance, humidity resistance and nonflammability enough to withstand severe use conditions in an engine room of an automobile, an OA equipment, a plant and an electrical appliance. The graded-refractive-index optical plastic material of the present invention can be used not only as an optical fiber but also as a flat or rod lens. In such a case, by increasing or decreasing the refractive index from the center to the periphery, it can function as a convex lens or a concave lens.